The girl was found at the Valero Gas Station in the 800 block of S. DuPont Highway in New Castle shortly after 11:30 a.m. on Wednesday.
Police say 28-year-old Jennifer Holmwood of the 200 block of Oak Acres Drive in Middletown stopped there to get gas.
Holmwood then allegedly took the girl to a portable bathroom outside the gas station, returned to her vehicle, then drove off leaving the girl behind.
A truck driver and frequent customer of the station saw the child alone and took her inside where she was picked up by police.
The girl was taken to Troop 9 and, while there, troopers were called to respond to Lorewood Grove Road and Ratledge Road in Middletown where Jennifer Holmwood's vehicle had become disabled.
A passing motorist stopped to assist Holmwood and, police say, Holmwood told that driver she could not find her daughter.
Jennifer Holmwood was arrested and charged with Reckless Endangering and a fifth offense of DUI.
The little girl was turned over to family members. She was not injured.